The present invention relates generally to methods and apparatuses for solidifying molten metal and more particularly to methods and apparatuses for doing so employing rheocasting. Rheocasting, also known as slurry casting, is a procedure in which molten metal is subjected to vigorous agitation as it undergoes solidification. Absent such agitation, dendrites would form as the metal solidifies. A dendrite is a solidified particle shaped like an elongated stem having transverse branches extending therefrom.
Vigorous agitation converts the normally dendritic microstructure of the solidifying metal into a non-dendritic form comprising discrete, degenerate dendrites in a liquid matrix. The agitation, which may be either mechanical or electromagnetic, shears the tips of the solidifying dendrites, and this produces a metal slurry composed of relatively fine, spheroidal, non-dendritic particles or grains in a liquid matrix.
The rheocast material is typically fully solidified, then reheated to a semi-solid state temperature, and then subjected to forming under pressure, e.g. die forming. When the material is in a semi-solid state, it has a microstructure composed of solid particles in a liquid matrix.
It is desirable that there be a relatively fine grain size when metallic material is formed under pressure while in a semi-solid state. Fine grains or particles flow more readily than do coarse grains during forming under pressure in a semi-solid state. For example, one desirable steel microstructure for semi-solid forming has an aim austenitic grain size, when in a solid state, of no greater than about 150 microns.
A procedure in which molten metallic material is solidified by rheocasting and then reheated to a semi-solid state followed by forming under pressure is disclosed in Young U.S. Pat. No. 4,565,241. This patent discloses maintaining, within a specified range, the ratio between (a) the shear rate of the metal undergoing agitation and (b) the solidification rate of that metal. Doing so produces certain desired results from the standpoint of microstructure and forming costs. Either mechanical or electromagnetic agitation are contemplated.
The shear rate obtained with mechanical agitation may be ascertained with reasonable accuracy. However, that is not the case when electromagnetic agitation is employed; in such a case, complex mathematical models are required to calculate the shear rate. These models require one to estimate the viscosity of the metal undergoing rheocasting, and that viscosity depends largely upon the proportion of solid phase in the metal undergoing rheocasting. The proportion of solid phase can vary from 0 to 80%, and over that range of solid phase, the viscosity can vary over several orders of magnitude. As a result, the calculated value of the shear rate can vary over several orders of magnitude depending upon the estimated viscosity of the metal undergoing rheocasting.
Another consideration involved in the electromagnetic stirring of molten metal undergoing rheocasting is the efficiency with which the electromagnetic field is employed. Rheocasting typically employs a casting mold having open upstream and downstream ends, and rheocasting can be a continuous type of casting. Copper alloys having high thermal conductivities are the only materials that have been found suitable for constructing molds employed in the rheocasting of metals such as steel. The lower conductivities of other materials cause excessive thermal distortion. However, the electrical conductivities of copper alloys are almost directly proportional to their thermal conductivities. As a result, when a rheocasting mold is made from materials conventionally employed for that purpose, there is produced a very effective shield to electromagnetic stirring fields.
To overcome this shielding effect, it has been conventional to use electromagnetic stirring fields with a frequency of 10 Hertz or less when stirring steel in a continuous casting mold. However, with such low electromagnetic stirring frequencies, the angular velocity of the molten metal within the mold is relatively low, e.g. no greater than about 10 revolutions per second. In rheocasting, it would be desirable to use an electromagnetic stirring field having frequencies of 30 to 60 Hertz, preferably at the upper end of that range.
In the rheocasting of steel, the molten steel can form a continuous column of liquid many meters long. Generally, an electromagnetic stirrer will extend over only a small portion of the liquid column. The stirring effect of such a device will extend up to 15 diameters upstream and downstream of the stirring device due to secondary recirculating flows. Primary circulatory flow occurs in planes transverse to the axis of the column of molten metal, while secondary recirculating flows occur in planes transverse to the planes in which primary circulatory flow occurs. The secondary flows will absorb about half of the stirring energy introduced into the metal column and thus reduce the maximum rotational or angular velocity that can be imparted to the material undergoing agitation. Therefore reducing the secondary recirculating flows is a desirable aim because it conserves the stirring energy available from primary circulatory flow.
Another problem which can arise in a rheocasting process is the occurrence of hangers. A hanger is a solidified peripheral skin which hangs up on the walls of the casting mold or confinement chamber in which solidification begins, rather than moving downstream at the same rate as the rest of the metal undergoing solidification. This can result in a breakout at the outlet of the casting mold, i.e. molten metal leaking through the skin of the partially solidified metal.
A third problem which can arise in a rheocasting process is the occurrence of a columnar, dendritic zone at the periphery of the casting. This peripheral, columnar, dendritic zone has a structure that is unsuitable for forming in the semi-solid state and thus reduces the yield of rheocast feedstock obtained from the rheocasting process.